DealRay isn't bad, I'm trying it now. They send you a text whenever there's a bargain deal. Right now I just got a text about Frontier Deal:

Frontier Spring Sale: Fly Either Way Between NYC-MIA for $69, Chi. ORD-LAX or -Vegas for $79, Wash. IAD to West Palm for $39 and More!

Frontier Spring Sale

Right now Frontier is having a spring sale for a variety of destinations (44 cities) that they fly to for flights booked by March 5th and flown March 9th-April 29th. These could get you a cheap spring break getaway or save on flights to a cool conference in March or April.

Some highlights below, or check the link and select your city from the dropdown to book.

Fares are one-way, good in either direction.
New York LGA to/from:
Miami MIA - $69
Denver DEN - $79

Chicago ORD to/from:
Los Angeles LAX: $79
Las Vegas LAS: $79
Phoenix PHX: $58.10
Austin AUS: $59
Tampa TPA: $49
Miami: $69
Orlando: $69

Washington IAD to/from:
West Palm Beach, FL PBI - $39
Ft. Myers, FL RSW - $39
Chicago ORD - $39
Orlando MCO - $49
Atlanta ATL - $49
Las Vegas LAS - $79

Los Angeles LAX to/from:
Denver DEN - $59
Chicago ORD - $69

San Francisco SFO to/from:
Denver DEN - $69

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Also, follow certain handles on Twitter is a good technique.